Let me start off this blog post by saying we do have a legitimate severe weather threat on Wednesday, with the primary threat being damaging winds with gusts from 60 to 80 mph possible. Because of this you may have been hearing the term “derecho” quite a bit. In my opinion, this is only capitalizing on fear from last years very RARE storm. Let me tell you what’s really in the cards…..
1) Conditions are similar to last June, but forecasting a derecho days ahead of time is nearly impossible. By definition, a derecho is complex line of thunderstorms that travels a minimum distance of 240 miles (~400 km) or more, and produces a nearly continuous and widespread swath of damaging winds over that distance, with concentrated areas of wind speeds over 58 mph (93 km/hr). Surface wind gusts accompanying a derecho can often approach or exceed 100 mph. On June 29th of last year, there were over 1,000 reports of wind damage across the path of that storm! That is EXTREMELY rare! As a matter of fact, we only see one derecho in this part of the country on average once per year. Here is a breakdown by the NWS.
2) For a derecho to sustain itself you usually need between 4,000 and 6,000 j/kg of CAPE. This basically means you need a VERY warm and moist air-mass in place to sustain a derecho. For last years storm we had temps in the mid 90’s with about 5,000 j/kg of CAPE in place. This is a chart of the CAPE values on June 29th, 2012.
Tomorrow we are forecasting highs about 10 degrees cooler along with about 3,000 to 4,000 j/kg of CAPE. Don’t get me wrong, numbers like that are alarming, however they are not as high as last year.
3) We’ll probably see one or two Mesoscale Convective Complexes (MCS’s) roll through on Wednesday. Basically an MCS is a large cluster of storms that form in regions of high instability (warm and moist air) and are most common into the summer months riding along warm fronts. Short term models are showing this with one just to our North during the late afternoon hours on Wednesday (see below).
And another going right through West Central Ohio into the early morning hours Thursday.
PLEASE note that until the actual MCS forms, the path of these storms are VERY difficult to forecast. Damaging winds and the possibility of heavy rain over one inch are a BIG concern for me Wednesday/Wednesday night. Isolated tornadoes can’t be ruled out either! I’ll be sure to keep you updated tonight on Your Hometown Lima Stations for all the latest developments!